The present invention relates to a composite substrate which has excellent heat-dissipating properties (or heat-absorbing properties) and is suitable for a substrate with a semiconductor device generating heat mounted thereon. The present invention also relates to a semiconductor apparatus employing the composite substrate.
Recently, with the development of large-scale semiconductor devices and the increase of the integration density, the amount of heat generated from the semiconductor devices has increased. To prevent thermal distortion, the substrate, on which the semiconductor device generating heat is mounted, (hereinafter, referred to as "heat-generating semiconductor device") must have a small thermal expansion coefficient close to that of the semiconductor device. To increase heat dissipation, the substrate must have an excellent thermal conductivity.
As a material for the substrate having a small thermal expansion coefficient, a ceramic material and a metal material are known. Examples of the ceramic material include alumina, Forstellite, mullite, and the like. Examples of the metal material include an Fe--Co alloy such as Koval, a Ni alloy such as 42 Alloy, and the like. However, these materials have a problem in that they have low thermal conductivities. On the other hand, as a material for the substrate having a high thermal conductivity, copper, copper alloys, aluminum, aluminum alloys and the like are generally known. However, these materials have a problem in that they have large thermal expansion coefficients.
Hence, the conventional semiconductor apparatuses employ a composite substrate which comprises a board having a small thermal expansion coefficient and a board having a good thermal conductivity, both being bonded with a solder having a low melting point. A heat-generating semiconductor device is usually mounted on the side of the board having a small thermal expansion coefficient of the composite substrate.
However, the conventional composite substrate has the following problems. Since the composite substrate is formed of two heterogeneous materials quite different in properties, high bonding strength is hardly obtained. Crackings or peelings are likely to take place at the bonding portion of the composite substrate when the bonding portion repeatedly receives thermal stress due to heat generation from the semiconductor device. As a result, the heat dissipation properties are deteriorated. Furthermore, since the board, on which the heat generating semiconductor device is mounted, is poor in thermal conductivity, the heat dissipation properties of the composite substrate is low.